High resolution imaging instrument

ABSTRACT

An imaging instrument includes plural spaced-apart photon collectors, whose sampled outputs are correlated in pairs to yield brightness line integrals across a remote object being imaged. These integrals can be subject to matrix decomposition to yield a 2D array of image data corresponding to the object. Another imaging instrument includes plural spaced-apart reflectors, each having an optical fiber end at its focal point. The precise positions of the optical fiber ends are controlled by a control system (e.g., including piezo-electric positioners) that can be operated to counter-act the effect of atmospheric turbulence. The other ends of the fibers terminate at an image plane and serve to provide an output image.

RELATED APPLICATION DATA

[0001] This application is a continuation of application Ser. No.09/507,877, filed Feb. 22, 2000, which is a division of application Ser.No. 09/021,853, filed Feb. 11, 1998 (now U.S. Pat. No. 6,028,300), whichclaims benefit of application Ser. No. 60/037,541, filed Feb. 11, 1997.Application Ser. No. 09/507,877 is also a continuation-in-part ofapplication Ser. No. 08/875,505, filed Jul. 29, 1997, which is the U.S.nationalization of international application PCT/US95/01201, filed Jan.30, 1995. Application Ser. No. 08/875,505 is also a continuation ofapplication Ser. No. 08/171,661, filed Dec. 20, 1993 (now U.S. Pat. No.5,448,053), which is a continuation-in-part of application Ser. No.08/024,738, filed Mar. 1, 1993 (now U.S. Pat. No. 5,412,200). Priorityis claimed to these prior applications under 35 USC Section 120, and theissued patents are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to astronomical imaging, and moreparticularly relates to methods and apparatuses for astronomical imagingusing multiple sensors.

BACKGROUND AND SUMMARY OF THE INVENTION

[0003] In accordance with one aspect of the present invention, anastronomical imaging array is formed of several widely spaced photoncollectors (e.g. photodiodes). Each collector has associated with it adigitizing sampler that collects a stream of sample data from thephotodiode in response to a trigger signal provided by a time source.Samplers at different photon collectors are triggered at differentinstants in accordance with their spacing, and their relative opticalpath differences from the object being imaged. In particular, eachsampler is triggered to collect a record of samples when a given phasefront of light from the object being imaged is expected to pass thephoton collector.

[0004] Using intensity interferometry techniques, the sampled data fromeach photon collector is correlated with data from other collectors,yielding a waveform whose individual values represent a brightness lineintegral through the object. Using different photon collector pairs,different sets of brightness line integrals through the object areproduced. Matrix algebra is then employed to synthesize the collectedset of line integrals into a two dimensional image representing thebrightness of the object being imaged.

[0005] Intensity interferometry was invented by Hanbury Brown and Twiss,and is exemplified by the following articles (all by Hanbury Brown etal): “Correlation Between Photons in Two Coherent Beams of Light”(Nature, Vol. 177, pp. 27-29, Jan. 7, 1956); “A Test of a New Type ofStellar Interferometer on Sirius,” (Nature, Vol. 178, pp. 1046-48, Nov.10, 1956); “The Stellar Interferometer at Narrabri Observatory; I: ADescription of the Instrument and the Observational Procedure” (MonthlyNotices of the Royal Astronomical Society, Vol. 137, pp. 375-392, 1967);and “The Angular Diameters of 32 Stars,” (Monthly Notices of the RoyalAstronomical Society, Vol. 167, pp. 121-136, 1974). (These articles, andothers landmarks in the field of interferometry, are reprinted in therecent volume “Selected Papers on Long Baseline Stellar Interferometry,”SPIE Milestone Series, Vol. MS 139, 1997, edited by Lawson.)

[0006] No one, to our knowledge, has employed Hanbury Brown-Twiss (HBT)techniques as the basis for an imaging instrument. Instead, astronomicaluse of such techniques (for forty years) has been limited to thedetermination of stellar diameters.

[0007] We believe one factor contributing to the failure of others toemploy HBT techniques in imaging applications may be certaininterpretations that are traditionally accorded the original HanburyBrown papers. We note some possible alternative interpretations that maybe central to the functioning of the below-described embodiments.

[0008] One area in which HBT's work may have been misunderstood is thecommon impression that intensity interferometry requires that the twosensors be equidistant from the object being measured (c.f. section 3.3of their 1967 paper). HBT state that correlation diminishes about 10%with the first 1 nanosecond delay between the two collectors(corresponding to a path length difference of about one foot), anddiminishes exponentially as this delay is increased.

[0009] (To provide this equidistant spacing, the observatory at Narrabriemployed two detectors on a circular track having a radius of about 100m. By this arrangement, the two detectors could be placed equidistantfrom the star being measured, while also providing a range of spacingfrom 0-200 meters.)

[0010] We presently believe that intensity interferometry techniques canbe used with arbitrarily positioned detectors, including arbitrary threedimensional arrangements (e.g. one or more in space).

[0011] Another possible misinterpretation of HBT's original findings isthe impression that the variation in correlation, as a function ofdetector spacing, is a single-lobed function (c.f. FIG. 5 of HBT's 1967paper), trailing to zero at a normalized spacing of about 3.5. Webelieve it is likely that this function instead exhibits multiple sidelobes, of diminished amplitude, extending far beyond the 0-3.5 rangecontemplated by HBT. (Our belief is based, in part, on Fraunhoferanalysis of the stellar disk being imaged.) The existence of suchsecondary lobes would begin to suggest that detectors can be spaced muchfurther apart than previously thought possible.

[0012] We caution that our foregoing critiques of the HBT work arepreliminary but are believed to explain the operation of our detailedembodiment. Further study, however, may reveal other or additionalrationales.

[0013] Throughout our work we encounter wave/photon conundrums. Forexample, classical photon theory holds that a single photon can only besensed once, e.g., when it kicks an electron out of a valence band in aphotodiode. Thereafter, it ceases to exist. Yet HBT interferometry seemsto illustrate the contrary, by evidencing intensity correlations betweenspaced-apart optical detectors.

[0014] Hanbury Brown and Twiss acknowledged these conundrums, but posedno answer. In the intervening forty years, no satisfactory resolution ofthe conflicting photon and wave theories has been found. We offer none,and instead rely exclusively on classical wave theory in analyzingoperation of our system.

[0015] So that the present invention may be better appreciated, it maybe helpful to review other work in the field of astronomicalinterferometry.

[0016] Most astronomical interferometry traces its origins back toMichelson's work in the late 1800s. Michelson showed that light from asingle source, traveling different paths, can be combined to producefringe patterns. This is the principle on which modern radio telescopearrays work. For example, the New Mexico Very Long Array (VLA) includes27 antennas (each with a 25 m reflector) movably positioned within aY-pattern of up to 22 miles across. The data from each pair oftelescopes is combined (often after recording on tape with a timesynchronization signal) to form interference patterns. The structures ofthese patterns, and their changes with time (as the Earth rotates)reflect the structures of radio sources in the sky. By applying Fouriertechniques to the resulting interferometric patterns, conventionalimagery can be produced. (Such radio astronomy interferometry/imaging iswell detailed in extensive literature familiar to those active in thefield. A bibliography of writings on the topic can be found at theinternet addresshttp://sgra.jpl.nasa.gov/mosaic_v0.0/Spacevlbi_lib.html.)

[0017] The resolution of imagery obtained by such interferometrictechniques depends on the size of the array (the baseline). The fullyextended VLA, for example, has a resolution of 0.04 arcseconds at 43GHz. To obtain still better resolution, longer baselines are required.An example of a transcontinental baseline is the Very Long BaselineArray, which employs ten identical radiotelescopes spread from Hawaii tothe U.S. Virgin Islands.

[0018] A decade ago, first steps were made to extend interferometricbaselines still further—into space. During 1986-1988, the NASA TDRSSsatellite observatory—working in conjunction with ground-basedtelescopes in Japan and Australia—proved the feasibility of such systemsby recording interferometric fringes from six radioemitting sources.

[0019] Several space VLBI (very long baseline interferometry) projectshave since been proposed but have not been completed, among them the IVSand QUASAT programs. The costs and difficulties associated with placingcomplex optical telescopes into orbit played large roles in the demiseof these projects.

[0020] Just recently, the first fully-operational space VLBI projectbegan operation: the Japanese Institute of Space and AstronauticalScience's VSOP mission (“VLBI Space Observatory Program,” space antennadeployed Feb. 27, 1997, first fringes produced May, 1997). Anotherimminent program is the Russian RadioAstron mission, developed by theAstro Space Center of the Lebedev Physical Institute and scheduled forlaunch later in 1998. Each of these programs utilizes a single 8-10meter radio telescope in an elliptical Earth orbit, in conjunction withground radiotelescopes. Each observes in the 22, 5, and 1.6 GHz bands.(Imagery from, and information about, the VSOP mission is publiclyavailable on the world wide web at http://www.vsop.isas.ac.jp/.Information about the RadioAstron mission is publicly available at http://sgra.jpl.nasa. gov/mosaic_v0.0/RadioAstron.html.)

[0021] (The movement of an antenna in a space VLBI array, together withthe changing path length of the space-to-ground data link, complicatesthe array's operation. In particular, signals from the moving antennamust be temporally correlated with those from the ground telescopesbefore they can be combined to generate the interferometric data.However, such problems can be redressed by known techniques described inthe literature. To facilitate description of the present invention, anarray of fixed photon collectors is described—it being understood thattechniques borrowed from this space radiotelescope prior art can be usedto compensate for the dynamic effects introduced by placing one or moreoptical sensors into space.)

[0022] The interferometric principles employed in radio telescope arrayscan likewise be extended to arrays of optical telescopes. Recent effortsin the optical interferometry domain include the Cambridge OpticalAperture Synthesis Telescope (COAST) and the Sydney University StellarInterferometer (SUSI).

[0023] A further interferometric technique is amplitude interferometryas described, e.g., in Currie et al, “Four Stellar-Diameter Measurementsby a New Technique: Amplitude hiteferometry,” Astrophysical Journal,Vol. 187(1), Part 1, pp. 131,134 (Jan. 1, 1974). Amplitudeinterferometry is a variant of Michelson interferometry, designed tobetter measure stellar diameters in the presence of atmosphericturbulence.

[0024] All of the Michelson-based interferometry systems—whether radioor optical—rely on the wave conception of radiation, i.e. thatlight/radio waves exhibit localized maxima and minima which can becombined to constructively or destructively interfere and produce fringepatterns. This imposes on all such systems a high degree of physicalprecision (e.g. of sub-wavelength dimension) because received waves mustbe combined in known phase relationships in order for the resultingfringe patterns to have the desired meanings. At optical wavelengths,for examples, the lengths of connecting optical fibers must bephysically or synthetically matched to within millionths of an inch. Ifsuch arrays are extended to space (c.f. NASA's New MilleniumInterferometer: http://huey.jpl.nasa.gov/nmi/index.html), the locationsof the space sensors must be ascertained to the nanometer—aspecification which NASA acknowledges will require “a significantcapability enhancement.”

[0025] In contrast, the interferometry aspects of the present inventiondo not rely on these wave-based constructive/destructive interferenceforms of interferometry, with their attendant sub-wavelengthstolerances. Instead, HBT interferometry is concerned with thecorrelation of stellar intensity signals over time. The accuratemeasurement of time is a far easier task than the attainment andmaintenance of sub-wavelength physical tolerances. Moreover, the natureof correlation operations is forgiving of many temporal errors (i.e.sample records are time shifted as necessary to obtain optimumcorrelation, removing small errors in triggering times).

[0026] By eliminating the strict physical tolerances inherent inMichelson-based systems, costs are reduced and reliability is increased.Such detectors are thus well adapted for use in arrays whose baselinesextend into space. Like Michelson systems, arbitrarily fine angularresolutions can be obtained by spacing the detectors accordingly.

[0027] Also disclosed in the following specification, and sharing theattribute of plural spaced detectors, is a more traditional opticalimaging system (i.e. no interferometry). This system employs techniquesdescribed in the above-cited patents to characterize/model, atmosphericturbulence through which a telescope is seeing.

[0028] In accordance with this further aspect of the present invention,these turbulence modeling techniques are used in an array of widelyspaced small (e.g. three to twelve inch reflector) telescopes tocharacterize atmospheric turbulence above each telescope. (Eachtelescope is pointed at the same general region of the sky.) However,instead of using the resulting data to “unblur” each telescope'simagery, as in the patents' preferred embodiments, it is used todynamically reposition—at essentially real time (e.g. 100 Hzrepositioning)—the end of an optical fiber in each telescope's focalregion. Such fibers are run from the plural telescopes to a centralcollection facility, where their opposite ends terminate in an imageplane. This collected group of fiber terminations presents an image thatcan be viewed directly, or magnified/processed by traditional optics asdesired. (Alternatively, the image can be sampled/recorded several timesa second, and the resulting static images can be combined to synthesizea more accurate image.)

[0029] The foregoing arrangement yields extremely high angularresolution imagery (the exact resolution depends on the extent of thetelescope array) while employing inexpensive optical components (e.g.the small component telescopes).

[0030] The foregoing and additional features and advantages will be morereadily apparent from the following detailed description, which proceedswith reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a schematic block diagram of an intensity interferometryimaging system according to one embodiment of the present invention.

[0032]FIG. 2 is a diagram showing the geometrical relationship between apair of detectors, and a line integral across the object being observed,in the system of FIG. 1.

[0033]FIG. 3 is another view showing how a line integral relates tofeatures on the object being imaged.

[0034]FIG. 4 is a schematic block diagram of another multi-sensorimaging system according to another embodiment of the present invention.

DETAILED DESCRIPTION

[0035] Referring to FIG. 1, an intensity inteferometry imaging system 10according to one embodiment of the present invention includes pluralspaced-apart photon detection subsystems 12, and a data processingsubsystem 14.

[0036] Each photon detection subsystem includes a reflector 16, a sensor18, an amplifier 20, a high speed A/D converter (sampler) 22, a timesource 24, and a memory 26.

[0037] The reflector 16 in each illustrated photon detection subsystemis a parabolic mirror that serves to collect incident light and directit onto the sensor 18. The design of the mirror is not critical—itspurpose is simply to intercept as many photons as possible from thesource being imaged; high precision optics are not required. In theillustrated embodiment, the mirror has a one meter diameter. Smaller orlarger mirrors can, of course, be used.

[0038] The sensor 18 can take numerous forms, depending on theparticular application being served. In the illustrated embodiment, asimple photodiode is used.

[0039] The output signal produced by the sensor 18 is provided to anamplifier 20, which scales the signal preparatory to A/D conversion. Theillustrated amplifier has a bandwidth of 100 MHz. Such a high bandwidthamplifier is most important for high intensity sources, such as our sun,or for implementations employing very large photon collection mirrors.When imaging fainter objects, or using smaller mirrors, the bandwidth ofthe amplifier is less critical (e.g. a 10 MHz bandwidth amplifier willsuffice for most applications).

[0040] The output of amplifier 20 is provided to the input of a fast A/Dsampler 22. A 1 GHz digital oscilloscope (available from Tektronix) isused in the illustrated embodiment; many other devices can likewise beused. The digital oscilloscope produces 8-bit output samples,quantifying the instantaneous amplifier output on a scale of 0-255. 16-or 32-bit sampling devices can of course be used.

[0041] The digitized output samples from the sampler 22 are written intomemory 26. Due to the 1 GHz sample rate, data is provided to the memory26 at a rate of 1 megabyte per millisecond, so a fairly large memory isdesirably employed. A 32 megabyte memory, for example, can store a totalof 32 million samples (spanning a total sampling time of 32milliseconds).

[0042] The digitizing operation of sampler 22, and the writing of datainto memory 26, is initiated by a trigger signal provided to atriggering input of the digital oscilloscope. This triggering signal isprovided from the time source 24.

[0043] Digressing a moment, a quasi-hypothetical wavefront of light fromthe astronomical object being imaged will reach different of the photondetection subsystems 12 at different times, depending on their physicallocations. (“Quasi-hypothetical” simply pays homage to the equivocationsurrounding the concept of an optical wavefront.) The purpose of thetime source 24 is to synchronize the acquisition of sample data at eachof these detection subsystems relative to a common wavefront.

[0044] For example, if two detection subsystems 12 are positioned akilometer apart on a north-south line, and are used to image a star 45degrees above the horizon, due south, the relative geometry indicatesthat light from the star will reach the northern detection subsystem2.35 microseconds after it reaches the southern detection subsystem. Ifthe time source 24 at the southern detection subsystem is set to triggerits data acquisition at time T, the time source at the northerndetection subsystem should be set to trigger its data acquisition attime T+2.35 microseconds.

[0045] In the illustrated embodiment, each sampler 22 is triggered suchthat the center of its data record coincides with the arrival of asingular quasi-hypothetical optical wavefront emanating from the centerof the object. In simpler terms, each digitizer is triggered to end halfa record length time after the arrival of the singular wavefront,producing half the data record before arrival and half after arrival.Precision and accuracy issues produce some small variations about thisideal “half.”

[0046] In the preferred embodiment, the triggering time for eachdetection subsystem is programmed from a central control station 28(which typically houses the data processing subsystem 14), and isrelayed to the subsystems by telephone or wireless data communication.Sampling then begins when the local time source reaches the specifiedtime. (The end of sampling is also controlled by a signal from the timesource, a programmable interval after the sampling starts.) In otherembodiments, direct control of the triggering operations can be effectedfrom the central station 28, e.g. through matched fiber optic lines. Insome applications, Doppler-compensated triggering times may be used.

[0047] Time sources 24 in the illustrated embodiments are GlobalPositioning System receivers, designed to receive time data from thearray of 26 GPS satellites in low earth orbit. The time signalsavailable to civilians from the GPS satellites are accurate only toabout 100 nanoseconds. Authorized government users can decode the timesignals to produce data accurate to within 10 nanoseconds. Accuracybetter than the 100 nanosecond civilian standard can be achieved byaveraging several intervals of the 100 nanosecond-accurate clocksignals.

[0048] Although FIG. 1 shows just a few photon collection subsystems 12,in actual practice dozens, hundreds, or thousands of such subsystems areemployed. Also, the clipart of FIG. 1 suggests a fairly sophisticatedcollection system. In practice, simpler is generally better. The maincriteria for the reflector/sensor are (1) collecting as many photons aspracticable, and (2) ensuring—to the degree practicable—that lightreceived by a sensor emanates only from the object being imaged.(Realistically, it suffices if light is collected from a smallneighborhood around the object being imaged.) Desirably, thereflector/sensor should also permit tracking of the target object as theearth rotates, although this is not essential.

[0049] Generally speaking, no spectral filtering is performed; everyphoton helps. (There are situations in which limited filtering may beappropriate. An example is the differential phase shifts that differentwavelengths of light may experience as they travel through differentcolumns of the earth's atmosphere to different of the photon collectionsubsystems. Such vagaries may be reduced by filtering out certainwavelengths prior to the sensor.)

[0050] The photon detection subsystems 12 can be sparsely arrayed inrelative random placements, or systematically arrayed, or somecombination—driven as much by the randomness of human population centerlocations as anything else, with typical pair-distances being anywherefrom a few hundred meters to thousands of kilometers or much, muchlarger still.

[0051] It is desirable to have extremely long record lengths in thesamplers 22, with relatively fast transfers to mass storage,facilitating high repetition rates in the acquisition of finite segmentwaveforms. In an illustrative embodiment, 5,000-10,000 samples areacquired each time a photon collection subsystem 12 is triggered.Desirably, tens, hundreds, or thousands of such data records areacquired within a brief interval. This interval should generally be asshort as possible, usually less than a second. (Depending on theprocessing capabilities of the photon detector subsystem, a thousand10,000 sample records might be acquired in 50 milliseconds or less.)These extras sets of waveforms are collected in order to increase thesignal to noise ratio in the ultimate resulting image, assuming that thetotal collecting time is short enough so that the object changesinappreciably.

[0052] The data from the memories 26 is transferred to the dataprocessing subsystem 14 for post processing.

[0053] Data processing subsystem 14 is illustrated as including a memory30, a correlator 32, and a matrix decomposition element 34. Althoughdedicated hardware can be used for these latter two elements, otherembodiments implement these functions in software on a general purposecomputer.

[0054] Data records from each of the memories 26 is copied into memory30, which is suitably large for this purpose. Memory 30 desirablyincludes disk storage for storing the complete set of input data,together with a large random access memory for storing data recordscurrently being processed.

[0055] Correlator 32 performs standard correlation operations betweendata records produced by selected pairs of photon detector subsystems12. Such correlation, when applied to any pair of waveforms produced bythe same optical wavefront, results in a new waveform whose individualvalues each represents a total brightness line integral through theobject, where the line in question is perpendicular the line axis drawnbetween the given pair of detector subsystems. This will be more readilyapparent by reference to FIGS. 2 and 3.

[0056]FIG. 3, for example, shows the correspondence between a star 40being imaged, and a brightness line integral 42 (not to scale)corresponding thereto. Line integral 42 is produced by correlator 32 inresponse to data records produced by a pair of photon detectorsubsystems whose axis is parallel to line X-X through the star 40.Different vertical “bands” 44 are illustrated through star 40, witharrows 46 showing the portions of the line integral 42 correspondingthereto.

[0057] If star 40 were of uniform brightness across its extent,brightness line integral 42 would be a smooth, sinusoidal function,beginning at zero at the end of the disk, growing to a maximum value,and then tapering off to zero at the opposite end of the disk.

[0058] In reality, the star 40 is not of uniform brightness. Theillustrated star, for example, has dark splotches 48 (e.g. sunspots).These irregularities in the star's brightness are manifested asirregularities 50 in the brightness line integral 42.

[0059] It will be recognized that the positions of irregularities 50 inthe brightness line integral 42 indicate one dimension of the splotches'position on the stellar disk (i.e. the left/right position in thefigure). However, line integral 42 does not provide any data about thesplotches' position in the other dimension (up/down). But by correlatinga different pair of data records, from a pair of detection subsystemsoriented along a different axis, a different brightness profile can beobtained, e.g. one along line Y-Y. Similarly, yet another pairing canprovide brightness data along line Z-Z, etc.

[0060] By processing the different line integrals produced by correlator32, along different axes through the object being imaged, the positionof surface features on object 40 can be determined. While two orthogonalline integrals theoretically contain all the information needed to fullyresolve the two dimensional image brightness profile, in practice, manysuch brightness line integrals are collectively processed to obtain thisdata.

[0061] The collective processing of the different brightness lineintegrals to yield the two dimensional brightness data is performed bythe matrix decomposition element 34. It will be recognized that thebrightness value at any position (x,y) on the object 40 can bedetermined by examining the projection of point (x,y) onto each of thedifferent line integrals 42 (i.e. along line X-X, along line Y-Y, alongline Z-Z, etc.), and solving the associated linear equations todetermine what brightness value would yield the observed values at eachof the projected points.

[0062] As noted earlier, each time source has some inherent error (e.g.1-100 nanoseconds). However, when correlating thousands of sets of datasamples, a self consistency about where each waveform exists relative tothe others emerges. This self-consistency allows the sets of datasamples to be matched to within one sample period (i.e. 1 nanosecond).The time error associated with each set of data samples can then bedetermined.

[0063] The post-processing operations detailed above were expressed inthe earlier-filed provisional application as follows:

[0064] A section of the correlated waveform, generally centered in thedata record to some degree of accuracy, thus contains a set of such(contiguous) line integral values, essentially covering the object intotal, with some overlap between line integrals related to the samplingrates of the digitizers. Except in extremely high resolution imagingwhere slight dynamic rotation of the axes are often desired (as in verydistant object imaging), the collected set of correlation waveforms fromthe, e.g. one minute set, can be added to effect a higher signal tonoise ratio set of covering line integral values. It can be appreciatedthat a second pair of detectors will produce a second set of coveringline integrals in the identical fashion, with, in general, a differentorientation and spacing (spacing being a function of the lateraldistance between photodetectors and the sampling rates of the digitizers[here assumed equal]). Likewise, a third pair and a fourth pair and afifth pair and so on each produce a generally unique set of lineintegral values, until all combinations of pairs are exhausted.

[0065] The entire set of line integrals thus represents a rather simplesystem of linear equations (with certain basic “bias” differencescompensated for) over any garden variety set of image basis functions(such as square pixels), a system whose solution produces an image ofthe object where the resolution (“pixel spacing”) of the resulting imageis generally equivalent to the breadth of a line integral provided thereare sufficient numbers of pair combinations and sufficient rotationorientation randomness to ensure a full independent set of linearequations. For example, 50 scattered photodetectors with sufficientlylarge separations should be capable of producing a 1K by 1K image.

[0066] It will be recognized that imaging system 10 provides highresolution imagery, with extremely simple sensors. Moreover, the dataprocessing needed to generate the imagery is straightforward and doesnot involve, e.g., the Fourier (or other domain) transformationsrequired in radio astronomy and the like.

[0067] Turning now to FIG. 4, a different form of spaced-sensorastronomical imaging system is shown.

[0068] The FIG. 4 system 60 includes plural telescopes 62, each equippedwith a system to characterize the atmospheric distortion through whichthe telescope is viewing. Such a system is disclosed in theearlier-cited patents. The outputs from the telescopes 62 are combinedby a central station 64 to produce the final image.

[0069] Desirably, each of the telescopes 62 is fairly small (e.g. lessthan a meter) so that the tips/tilts of received wavefronts are not toocomplex. (If large reflectors are used, more complex atmosphericdistortion results.) In an exemplary system, the reflectors are on theorder of the Fried distance, r_(o) or less.

[0070] The telescope outputs which make up the final image are nottwo-dimensional sets of image data. Rather, they are single mode opticalfibers 66 which couple light from a single point in the telescope'sfocal region. The fiber end at the telescope (typically planar) isadaptively moved within this focal region by a three-dimensional piezoelectric positioning system, in response to real-time data about theturbulence of the atmosphere through which the telescope is viewing(e.g. repositioned 100 times per second). This positioning systemmaintains the fiber end at the point where light from the source beingimaged is focused, regardless of how intervening atmospheric turbulencemay cause this point to shift.

[0071] More particularly, each telescope 62 includes a dichroic mirrorbetween the reflector and the fiber end. The mirror couples light into afast (e.g. 100 frame/second) camera using, for example, a 512×512 pixelCCD (e.g. of the sort described in U.S. Pat. No. 5,444,280, incorporatedherein by reference). Data from this CCD is used, in accordance with thedisclosure of the earlier-cited patents, to produce tip/tilt data for aregion, e.g., a half degree across. (A half degree of space usuallyincludes sufficient bright stars to provide meaningful tip/tilt data.)This tip/tilt data determines the movement of the fiber end.

[0072] At the central station, the other ends of these fibers areterminated in an image plane. This collected group of fiber terminationspresents an image that can be viewed, recorded, or processed as desired.For example, the image can be projected, using glass lenses, onto aviewing screen. Alternatively, the image can be sampled/recorded severaltimes a second, and the resulting static images combined to synthesize amore accurate image, e.g. by a weighted average or the like. (Weightingcan be advantageously employed because the turbulence characterizingsystem can assess the relative merit of each recorded frame by referenceto the turbulence at that instant. The weight given to any image framecan be inversely proportional to the degree of turbulence under whichthe image was produced.)

[0073] It is important that the optical path lengths from the objectbeing imaged, to the termini of the fibers at the central station, bematched. The fiber lengths coupling the telescope feed points to thecentral station can be made equal. But additional optical delay must beprovided to account for the differing path length from the object toeach of the telescopes.

[0074] This additional optical delay can be provided by various means.One suitable arrangement is to interrupt the fiber from each telescopeand interpose a fiber optic delay line. An arrangement much like a pairof cooperating rotary electrical switches can be employed. Inparticular, the fiber from the telescope can terminate at the peripheryof a rotatable disk. Radially arrayed about this disk are first ends ofplural delay lines. The disk can be rotated so as to couple light fromthe end of the telescope fiber into any of these delay lines.

[0075] A reciprocal arrangement is used on the second part of theinterrupted fiber. That end is routed to the periphery of a secondrotatable disk, which has the second ends of the optical delay linesradially arrayed therearound. The two disks are turned to couple lightinto, and out of, the desired one of the delay lines. (A variety ofoptical and mechanical considerations come into play in theimplementation of this system including, e.g., how to utilize indexmatching fluids between the fiber ends when the fiber ends arephysically movable. Such details are within the capabilities of anartisan in the optical fiber field.)

[0076] A similar arrangement can be employed in which the optical delaylines do not terminate radially at the periphery of the disk, but ratherare normal thereto and near the edge thereof. The interrupted telescopeoptical fiber can be terminated near the edge of the disk to couple intoand out of such delay lines.

[0077] In alternative embodiments, electro-optic techniques can be usedto switch the light through different delay elements. Still further, atsuch time as analog optical delay devices become commonplace (e.g.devices which control the refractive index of a media in response to acontrol signal), they can be employed to advantageous effect.

[0078] Starlight collected by the telescopes is desirably used to aid inmatching of the optical path lengths using, e.g., conventional Michelsoninterferometric techniques. Gross precision (e.g. on the order of amillimeter or a centimeter) is achieved by the physical components.Adjustments can then be made, e.g., by servo mechanisms which repositioneach of the telescopes 62 in three dimensions, to fine tune match theoptical path lengths.

[0079] As in the FIG. 1 embodiment, the telescopes 62 can be arrayed inany fashion over an arbitrarily large area. (Current single mode fibertechnology presently limits the distribution of the telescopes to amaximum distance of about 100 kilometers from the central station 64. Asimprovements are made to the fibers, further separations will becomepractical.)

[0080] Due to the wide distribution of the telescopes 62, the turbulenceof the atmosphere can be tomographically modeled over a similarly widearea. This information can be advantageously used, for example, byaviation authorities who can use it to direct aircraft to areas of lowturbulence. (Turbulence data will be spotty at low altitudes if thetelescopes are widely spaced, since the viewing cones of each will notoverlap until higher altitudes. If such a system is employed foraviation purposes, it would be desirable to have more closely spacedtelescopes around the periphery of the array. This would permitidentification of low altitude turbulence as it moves into (and out of)the area monitored by the array.)

[0081] From the foregoing, it will be recognized that system 60 providesreal-time imaging of astronomical objects using inexpensive componenttelescopes. The angular resolution provided by the system depends on thedimensions of the telescope array; exceedingly high resolutions arepossible.

[0082] Having described the principles of our invention with referenceto preferred embodiments and several variations thereof, it should beapparent that the embodiments can be modified in arrangement and detailwithout departing from such principles. Accordingly, we claim as ourinvention all such modifications as may come within the scope and spiritof the following claims, and equivalents thereto.

We claim:
 1. A high resolution imaging system comprising: a dataprocessing unit including a memory, a correlator, and a matrixdecomposition system; a time base; and plural spaced-apart sensorscoupled both to the data processing unit and to the time base; whereinthe time base associates time data with information acquired by thesensors, and the data processing unit correlates the information fromthe plural sensors using the correlator, and processes same using thematrix decomposition system to yield high resolution image data.
 2. Thesystem of claim 1 in which the correlator comprises a processorprogrammed in accordance with software.
 3. The system of claim 1 inwhich the matrix decomposition system comprises a processor programmedin accordance with software.
 4. A method of generating image datacorresponding to a remote object, comprising: acquiring a first set ofphoton data from the remote object using a first sensor; acquiring asecond set of photon data from the remote object using a second sensorspaced apart from the first; and correlating the first and second setsof photon data to yield a brightness line integral representing a scanof image data across said object, a direction of said scan being afunction of the relative locations of the first and second sensors andthe remote object.
 5. The method of claim 4 that includes repeating themethod with other pairs of first and second sensors, and processing allof the image data thereby produced to yield two dimensional image data.6. The method of claim 5 in which the processing includes using matrixalgebra to synthesize the scan image data from the respective pairs ofsensors to yield the two dimensional image data.
 7. The method of claim4 that further includes quantifying atmospheric distortion through whichthe remote object is being imaged, and processing the image data tocompensate for at least part of said distortion.